Other Monosaccharides
High-Yield Summary
- Galactose (from lactose) is trapped by galactokinase as galactose-1-phosphate, then converted (via galactose-1-phosphate uridyltransferase) into glucose-1-phosphate, which enters glycolysis or glycogen synthesis.
- Impaired galactose metabolism → galactose reduced by aldose reductase to galactitol, which accumulates and is toxic to the lens of the eye → cataracts.
- Fructose (from sucrose) is trapped by fructokinase (liver/kidney) as fructose-1-phosphate, then cleaved by aldolase B into DHAP + glyceraldehyde — both feed into glycolysis.
- Fructose enters glycolysis downstream of PFK-1, bypassing the pathway's main rate-limiting checkpoint — so fructose metabolism proceeds regardless of cellular energy state, which is why excess fructose intake strongly drives lipid synthesis.
Key Terms
- Galactokinase
- Traps galactose as galactose-1-phosphate using ATP.
- Galactitol
- Sugar alcohol formed from galactose via aldose reductase when galactose metabolism is impaired; toxic to the eye lens.
- Fructokinase
- Liver/kidney enzyme that traps fructose as fructose-1-phosphate.
- Aldolase B
- Liver-specific enzyme cleaving fructose-1-phosphate into DHAP and glyceraldehyde.
Galactose Route
- 1Lactose → glucose + galactose (lactase, in the intestine)
- 2Galactose → galactose-1-phosphate (galactokinase, uses ATP)
- 3Galactose-1-phosphate → glucose-1-phosphate (galactose-1-phosphate uridyltransferase + epimerase)
- 4Glucose-1-phosphate → glycolysis (via glucose-6-phosphate) or glycogen synthesis
Fructose Route
- 1Sucrose → glucose + fructose (digestion); both absorbed, carried to liver via hepatic portal vein
- 2Fructose → fructose-1-phosphate (fructokinase, liver/kidney)
- 3Fructose-1-phosphate → DHAP + glyceraldehyde (aldolase B)
- 4Glyceraldehyde → glyceraldehyde-3-phosphate; both fragments enter glycolysis (downstream of PFK-1), glycogenesis, or gluconeogenesis
Galactose vs. Fructose
| Feature | Galactose vs. Fructose |
|---|---|
| Source disaccharide | Lactose / Sucrose |
| Trapping enzyme | Galactokinase / Fructokinase |
| Key conversion enzyme | Galactose-1-P uridyltransferase / Aldolase B |
| Entry to glycolysis | Via glucose-1-P → glucose-6-P / DHAP + G3P (bypasses PFK-1) |
| Impaired-pathway consequence | Galactitol buildup → cataracts / Bypasses PFK-1 regulation entirely |
Common MCAT Trap
- Fructose entering glycolysis downstream of PFK-1 means it isn't subject to the cell's main energy-status checkpoint — don't assume all sugars are regulated equally.
- Aldolase B (fructose) is liver-specific; don't confuse with the general aldolase of glycolysis (cleaves F1,6BP, not F1P).
Quick Recall
What accumulates when galactose metabolism is impaired, and what does it cause?
Which enzyme cleaves fructose-1-phosphate, and into what?
Why does fructose metabolism proceed regardless of cellular energy state?